Superconducting control elements for RF antennas

ABSTRACT

Control elements for RF antennas including high temperature superconducting capacitors, alone or in combination with other elements, including high temperature superconducting films, structures, and applications are formed. In one embodiment a high temperature superconducting capacitor is coupled to an inductor to form a resonant circuit. In another embodiment a high temperature superconducting capacitor is used to make a low-resistance cross-over for an inductor coil. Additional circuits include circuits which do not use non-superconducting materials in the circuit, circuits which have coupled superconducting inductors to provide low-loss signal coupling, tuning and bandwidth broadening, and circuits which include switches to shut off the superconductivity of a superconducting element including low-loss photoconducting and superconducting thermal switches. These circuits may be used to improve Magnetic Resonance Imaging (MRI).

RELATED APPLICATION INFORMATION

This application is a continuation of application Ser. No. 07/934,921,filed Aug. 25, 1992, entitled “Superconducting Control Elements for RFAntennas”, now U.S. Pat. No. 6,335,622.

FIELD OF THE INVENTION

This invention relates to useful devices from high temperaturesuperconducting materials. Specifically, it relates to circuits havingsuperconducting capacitors and inductors, alone or in combination withother elements made from high temperature superconducting films and,optionally, including low-loss switches. This invention also relates tothe use of such circuits to improve Magnetic Resonance Imaging (“MRI”).

BACKGROUND OF THE INVENTION

Capacitors are basic building blocks for electronic circuits. Capacitorsfunction principally to store charge or to add reactance to an accircuit. When combined with other electronic devices, numerous usefulcircuits may be constructed. For example, when a capacitor iselectrically connected to an inductor (an electromagnetic field storagedevice) a resonant circuit results. Such resonant circuits have numerousapplications, such as for an antenna to pick-up radio frequencyradiation.

Superconductivity refers to that state of metals and alloys in which theelectrical resistivity is zero when the specimen is cooled to asufficiently low temperature. The temperature at which a specimenundergoes a transition from a state of normal electrical resistivity toa state of superconductivity is known as the critical temperature(“T_(c)”).

Until recently, attaining the T_(c) of known superconducting materialsrequired the use of liquid helium and expensive cooling equipment.However, in 1986 a superconducting material having a T_(c) of 30K wasannounced. See, e.g., Bednorz and Muller, Possible High T_(c)Superconductivity in the Ba—La—Cu—O System, Z. Phys. B-Condensed Matter64, 189-193 (1986). Since that announcement superconducting materialshaving higher critical temperatures have been discovered. Collectivelythese are referred to as high temperature superconductors. Currently,superconducting materials having critical temperatures in excess of theboiling point of liquid nitrogen, 77K at atmospheric pressure, have beendisclosed.

Superconducting compounds consisting of combinations of alkaline earthmetals and rare earth metals such as barium and yttrium in conjunctionwith copper (known as “YBCO superconductors”) were found to exhibitsuperconductivity at temperatures above 77K. See, e.g., Wu, et al.,Superconductivity at 93K in a New Mixed-Phase Y—Ba—Cu—O Compound Systemat Ambient Pressure, Phys. Rev. Lett. 58, No. 9, 908-910 (1987). Inaddition, high temperature superconducting compounds containing bismuthhave been disclosed. See, e.g., Maeda, A New High-Tc OxideSuperconductor Without a Rare Earth Element, J. App. Phys. 37, No. 2,L209-210 (1988); and Chu, et al., Superconductivity up to 114K in theBi—Al—Ca—Br—Cu—O Compound System Without Rare Earth Elements, Phys. Rev.Lett. 60, No. 10, 941-943 (1988). Furthermore, superconducting compoundscontaining thallium have been discovered to have critical temperaturesranging from 90K to 123K (the highest critical temperatures to date).See, e.g., G. Koren, A. Gupta, and R. J. Baseman, Appl. Phys. Lett. 54,1920 (1989).

These high temperature superconductors have been prepared in a number offorms. The earliest forms were preparation of bulk materials, which weresufficient to determine the existence of the superconducting state andphases. More recently, thin films on various substrates have beenprepared which have proved to be useful for making practicalsuperconducting devices. More particularly, the applicant's assignee hassuccessfully produced thin film thallium superconductors which areepitaxial to the substrate. See, e.g., Olson, et al., Preparation ofSuperconducting TlCaBaCu Thin Films by Chemical Deposition, Appl. Phys.Lett. 55, No. 2, 189-190 (1989), incorporated herein by reference.Techniques for fabricating and improving thin film thalliumsuperconductors are described in the following patent and copendingapplications: Olson, et al., U.S. Pat. No. 5,071,830, issued Dec. 10,1991; Controlled Thallous Oxide Evaporation for Thallium SuperconductorFilms and Reactor Design, Ser. No. 516,078, filed Apr. 27, 1990; In SituGrowth of Superconducting Films, Ser. No. 598,134, filed Oct. 16, 1990;Passivation Coating for Superconducting Thin Film Device, Ser. No.697,660, filed May 8, 1991; and Fabrication Process for Low LossMetallizations on Superconducting Thin Film Devices, Ser. No. 697,960,filed May 8, 1991, all incorporated herein by reference.

High temperature superconducting films are now routinely manufacturedwith surface resistances significantly below 500 μΩ measured at 10 GHzand 77K. These films may be formed into resonant circuits. Suchsuperconducting films when formed as resonators have an extremely highquality factor (“Q”). The Q of a device is a measure of its lossiness orpower dissipation. In theory, a device with zero resistance (i.e. alossless device) would have a Q of infinity. Superconducting devicesmanufactured and sold by applicant's assignee routinely achieve a Q inexcess of 15,000. This is high in comparison to a Q of several hundredfor the best known non-superconducting conductors having similarstructure and operating under similar conditions.

Superconducting thin film resonators have the desirable property ofhaving very high energy storage in a relatively small physical space.The superconducting resonators are compact and lightweight. Anotherbenefit of superconductors is that relatively long circuits may befabricated without introducing significant loss. For example, aninductor coil of a detector circuit made from superconducting materialcan include more turns than a similar coil made of non-superconductingmaterial without experiencing a significant increase in loss as wouldthe non-superconducting coil. Therefore, the superconducting coil hasincreased signal pick-up and is much more sensitive than thenon-superconducting coil.

Typical resonant circuits are generally limited in their application dueto their signal-to-noise ratios (“SNR”). For example, the SNR in apickup coil of a MRI detector is a limiting factor for low-field MRIsystems. Although the low-field MRI systems have a number of advantagesover high-field MRI (including cost, site requirements, patient comfortand tissue contrast), they have not yet found wide-spread use in theU.S. because, in part, of their lower SNR. Resonant circuits made fromsuperconductors improve SNR for low-field human imaging. Therefore, anappropriate superconducting resonant circuit, depending on the fieldlevel, coil type, and imaging region, will enable wide-spread use oflow-field MRI.

An MRI detector including a low temperature superconducting coil andcapacitor has been described. See, e.g., Rollwitz, U.S. Pat. No.3,764,892, issued Oct. 9, 1973. In addition, resonant circuits for useas MRI detectors which include high temperature superconducting coilsand non-superconducting capacitors have been described. See, e.g., Wang,et al., Radio-Frequency Losses of YBa₂Cu₃O_(7-δ) CompositeSuperconductors, Supercond. Sci. Technol. 1, 24-26 (1988); High Tc Usedin MRI, Supercond. Indus. 20 (Winter 1990); and Hall, et al., Use ofHigh Temperature Superconductor in a Receiver Coil for MagneticResonance Imaging, Mag. Res. in Med. 20, 340-343 (1991).

SUMMARY OF THE INVENTION

Resonant circuits made from high temperature superconductors enjoyincreased SNR and Q values. The devices of the present applicationinclude high temperature superconducting capacitors and inductors havingvarious structures. These capacitors and inductors may be used, forexample, in resonant circuits for use in MRI detectors.

The preferred embodiments of superconducting capacitors of the presentapplication comprise high temperature superconducting members separatedby a low-loss dielectric and may be an interdigital structure or a platestructure. Applications of these preferred embodiments utilize one orboth of these superconducting capacitor structures alone or incombination with other elements which may also be superconducting.

In one embodiment, a superconducting capacitor is fabricatedmonolithically on the same substrate as is an inductor. The circuit maybe completed by connecting gold contact pads. The capacitance can beeasily set by scribing away part of the capacitor and can be easilytuned by placing a dielectric or conductor on top of the capacitor. Thisembodiment may also include an additional superconducting capacitor as atuning capacitor which can be used to tune the original capacitor eitherby scribing the tuning capacitor or by positioning a dielectric orconductor on top of it.

Optionally, the signal may be coupled out of the resonant circuit usinga superconducting inductor.

In another embodiment, a circuit, which includes an interdigitalsuperconducting capacitor fabricated monolithically on the samesubstrate as an inductor, is completed by conducting cross-overs whichare built over the inductor.

In yet another embodiment, a circuit, which includes a superconductinginductor attached to two superconducting plates, is completed by asecond superconductor layer which also has two plates that formcapacitors with the plates in the first layer and, thus, complete thecircuit without using normal metal. The second layer can be addedmonolithically, by forming superconducting structures on both sides of adielectric. The second circuit can also be added by hybridizing togethertwo different superconducting structures, separated by a dielectric.

Accordingly, it is a principal object of this invention to provide hightemperature superconducting capacitors.

It is also an object to provide high temperature superconductingcapacitors which are tunable.

It is an additional object of this invention to use superconductingcapacitors and/or tunable superconducting capacitors in conjunction withinductors to provide circuits which are at least partiallysuperconducting.

It is a further object of this invention to provide circuits withsuperconducting capacitors and superconducting inductors.

It is another object of this invention to provide resonant circuitswhich are completed without using normal metal.

It is yet a further object of this invention to provide improved MRIcoils with high temperature superconducting components.

It is still an additional object of this invention to provide coupledsuperconducting inductor coils for improved reception of electronicsignals and for low-loss tuning of resonant circuits.

It is also another object of this invention to provide thermal switchesfor switching superconductor material between superconducting andnon-superconducting states.

It is also another object of this invention to provide photoconductiveswitches to provide low-loss switching in resonant circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of a superconducting capacitor having aninterdigitated structure.

FIG. 1B is a cross-sectional view of the capacitor of FIG. 1A.

FIG. 2A is a top view of a superconducting capacitor having a platestructure.

FIG. 2B is a cross-sectional view of the capacitor of FIG. 2A.

FIG. 2C is a cross-sectional view of a hybrid form of thesuperconducting capacitor of FIG. 2A.

FIG. 3 is a top view of an MRI coil showing an inductor coil, acapacitor, a tuning capacitor, and pads for a cross-over connection.

FIG. 4 is a top view of an MRI coil showing an inductor coil, acapacitor, and pads for a cross-over connection.

FIG. 5A is a top view of an MRI coil showing an inductor coil and twocapacitors.

FIG. 5B is a partial cross-sectional view of the MRI coil of FIG. 5Ataken along 5B—5B.

FIG. 5C is a partial cross-sectional view of an MRI coil showingcapacitor plates hybridized to a dielectric.

FIG. 6 is a side view of two interacting inductor coils.

FIG. 7A is a top view of a resistive control line thermal switch.

FIG. 7B is a top view of a focussed light source thermal switch.

FIG. 8A is a top view of a photoconductor switch system for shortcircuiting an inductor.

FIG. 8B is a top view of a photoconductor switch system for shortcircuiting a capacitor.

FIG. 8C is a top view of a photoconductor switch system for addingcapacitance to a circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to the drawings in more detail, FIGS. 1A and 1B and FIGS. 2A, 2Band 2C show respectively an interdigitated structure and a platestructure of superconducting capacitor 10 of the present invention. Asshown, the capacitor 10 comprises two superconducting members 11 a and11 b separated by a low loss dielectric 12 (e.g. LaAlO₃, MgO, sapphire,or polyimide).

The interdigitated structure shown in FIGS. 1A and 1B comprises acapacitor 10 having superconducting members 11 a and 11 b fabricatedmonolithically on one side of the same substrate 12. The members 11 aand 11 b each comprise a plurality of fingers 13 a and 13 b respectivelywhich extend on the surface of the dielectric substrate 12. The fingers13 a and 13 b are positioned on the dielectric 12 such that fingers 13 aare interspersed between fingers 13 b.

The interdigital superconducting capacitor 10 has the benefit of beingeasily tuned. It can be tuned either by scribing away part of asuperconducting member 11 a or 11 b or by placing a dielectric orconductor (not shown) on the surface of the capacitor 10.

The plate structure shown in FIGS. 2A, 2B, and 2C comprises a capacitor10 having superconducting members 11 a and 11 b on two sides of the samesubstrate 12. As shown in FIGS. 2B and 2C, the members 11 a and 11 beach comprise a plate of superconducting material positioned oppositeeach other on opposite sides of the dielectric 12. As shown in FIG. 2B,the superconducting members 11 a and 11 b may be fabricatedmonolithically on opposite sides of the same dielectric substrate 12. Asshown in FIG. 2C, the superconducting members 11 a and 11 b may befabricated separately on substrates 14 a and 14 b, respectively, andthen hybridized to opposite sides of the same dielectric 12.

As described above, one application of the superconducting capacitors 10of the present invention is incorporation into various resonantcircuits. For example, they may be incorporated into resonant circuitsfor use as MRI detector circuits. FIG. 3 shows a preferred embodiment ofan MRI resonant detector circuit 20. This circuit 20 comprises adielectric substrate 21 upon which the circuit is fabricated, aninductor coil 22 which may be made from superconducting material, asuperconducting capacitor 23 having an interdigitated structure, and asuperconducting tuning capacitor 24 having an interdigitated structure.The dielectric material used in the present invention preferablycomprises an LaAlO₃ wafer and the superconducting material used in thepresent invention preferably comprises an epitaxial thin film thalliumbased superconductor. Alternative dielectric materials which may be usedinclude magnesium oxide, sapphire, and polyimide. Alternativesuperconducting materials which may be used include yttrium based orbismuth based superconductors. Any substrate or superconductorconsistent with the structure and functionality of this invention may beused.

The dielectric substrate 21 comprises a two inch wafer (approximately 20mils thick) upon which a superconducting film is patterned to form thecircuit 20. The inductor coil 22 is a fifteen turn inductor coil with aline width of 200μ, a pitch of 400μ (i.e. a spacing of 200μ between eachline), and a total line length of 175.3 cm. The inductor coil 22 has amean radius of 1.86 cm, an outer diameter of 4.3 cm, and an innerdiameter of 3.14 cm.

The superconducting capacitor 23 comprises an interdigital structurehaving a total of twenty-one interspersed tines each having a line widthof 50μ and a pitch of 75μ (i.e. a spacing of 25μ between each tine).

To trim or set the capacitance of the capacitor 23, the tines of thecapacitor 23 may be cut or a dielectric or conductor may be placed onits surface. The capacitor 23 has a mean radius of 1.3 cm and an outerdiameter of 2.8 cm.

The tuning capacitor 24 comprises an interdigital structure having atotal of ten interlocking tines each having a line width of 200μ and apitch of 400μ (i.e. a spacing of 200μ between each tine). The tuningcapacitor 24 allows for dynamic tuning (i.e. tuning during operation) bysliding a dielectric or conductor across its surface.

In addition, gold contact pads may be provided on the ends 25 a and 25 bof the inductor coil 22 and the superconducting capacitor 23respectively for providing points at which the circuit may be completed.It is highly desirable to keep the contact resistance to a minimum.Placing the gold on the superconductor and annealing it helps improvethe bond between the gold and the superconductor. In addition, placingthe gold on the superconductor prior to placing photo-resist on thesuperconductor helps reduce the contact resistance. See copendingapplication Fabrication Process for Low Loss Metallizations onSuperconducting Thin Film Devices, Ser. No. 697,960, filed May 8, 1991,assigned to a common assignee, incorporated herein by reference. Othertechniques to reduce contact resistance, such as annealing, may beadvantageously employed.

For example, a hybrid high temperature superconductor crossovercomprising a line of high temperature superconductor film patterned anddiced from a separate dielectric wafer and having gold contact pads maybe glued on the coil surface and the gold contact pads of the crossovermay be connected to the gold contact pads of the circuit with goldwire-bonds thereby connecting the inductor 22 and capacitor 23 in aparallel RLC configuration. This arrangement has the benefit ofminimizing the amount of normal metal (as opposed to superconductingmetal) required for the crossovers. Additional steps may be taken tominimize the amount of normal metal used in these circuits includingusing indium bumps to connect the circuit to another coil. In the indiumbump configuration a superconducting crossover, as described above, isflipped over and its pads are electrically connected to the pads on thecircuit using indium bumps.

FIG. 4 shows a second resonant circuit 30 having an inductor coil 31, aninterdigitated superconductor capacitor 32, and pads 33 for connectingcross-overs. This circuit 30 differs from the circuit 20 described abovein that the lines of the inductor coil 31 are configured to provide anarea 34 on the inductor where the width of the inductor is narrowed.This narrowed width is beneficial because it provides for shorter and,therefore, less resistive crossovers.

The crossovers of the resonant circuits of the present invention areresistive and, therefore, lossy because of the normal metal used tocomplete the circuits (e.g. gold contact pads). The less normal metalused in these circuits, the less loss the circuit experiences (i.e. thehigher the efficiency of the circuit). By narrowing the width over whichthe crossovers must span, one can minimize the length of the crossoversand thereby provide for more efficient crossovers. The circuit shown inFIG. 4 shows a circuit embodying these principles.

FIGS. 5A and 5B show a third resonant circuit 40 having a superconductorcapacitor of the present invention. The circuit 40 shown comprises adielectric substrate 41, an inductor coil 42, two upper superconductingplates 43 a, and two lower superconducting plates 43 b. The platescouple through the dielectric to form two capacitors which complete thecircuit by crossing over the inductor without using normal metal. Thus,a high-Q resonant structure can be formed from a two-sided film (asshown in FIGS. 5A and 5B). This same result could be achieved by using ahybrid structure where the inductor 42 and plates 43 a comprise one hightemperature superconducting film and the plates 43 b comprise a secondhigh temperature superconducting film which are held near each otherseparated by a low loss dielectric 41 (as shown in FIG. 5C). Theseconfigurations provide circuits which do not incorporate anynon-superconducting materials and, therefore, do not experience loss dueto use of non-superconducting materials.

FIG. 6 shows an application of superconducting inductor coils. Asuperconducting inductor may be magnetically coupled to the inductor ofa resonant coil and, thus, allow for tuning and for signal detectionwithout the use of lossy normal metal wires. Magnetic flux links bothcoils and causes them to interact. If both coils are superconducting,the structure gives coupling and tuning with low loss (i.e. high Q). Inaddition, the band-width of the coils can be changed by changing thegeometry (e.g. the spacing and the number of turns) between the two(i.e. by changing the mutual inductance). Low-loss resonant circuitshave high Qs and are, thus, sensitive only to narrow signal bandwidths.In some applications it will be important to detect a given bandwidthwith very low loss; the method of coupled superconducting inductorsdescribed above is one way to achieve a desired bandwidth with low loss.

In an application, as shown in FIG. 6, a first coil 50, a resonant coil,which may be a part of an MRI detector coil and a second coil 51, asimple inductor, are provided. The second coil 51 tunes the detectorcoil 50 and provides means for bringing out the signal from the detectorcoil 50 without soldering wires to it.

In addition, the circuits described may include various switchingdevices to “de-Q” or shut off the superconducting capability of thecircuit or an element of the circuit, or to shift the resonant frequencyof the circuit. For example, a thermal switch could be used. A thermalswitch de-Qs a superconductor by heating the superconductor at one ormore points to a temperature above its T_(c). Once heated above itsT_(c), the superconductor loses its superconductivity. By heating asufficient amount of the circuit above T_(c), the resistance of thecircuit can be increased enough to sufficiently de-Q the resonance.

FIGS. 7A and 7B show two embodiments of thermal switches. As shown inFIG. 7A, a resistive control line 61 is placed near a line ofsuperconducting material 60. Running current through the resistivecontrol line 61 causes it to heat up. As the resistive control line 61heats up, some of the heat is transferred to the superconductingmaterial 60 which then is heated to a temperature above its T_(c). FIG.7B shows a thermal switch where the heat source is a light source 62which is then focussed on a particular area 63 of a superconductingmaterial 60. Once the superconducting material 60 is heated above itsT_(c), the material is no longer superconducting.

Another switch, shown in FIGS. 8A, 8B, and 8C, uses a photoconductor 70which is switched between conducting and non-conducting states byexposing it to light. The photoconductor 70, for example, can bepositioned on or near a superconducting inductor 71 or a superconductingcapacitor 72 such that when the photoconductor 70 is in a conductingstate it de-Qs the resonant circuit by shorting out the inductor 71 orcapacitor 72 (see FIGS. 8A and 8B) or by adding inductance orcapacitance (see FIG. 8C). Alternatively, the configurations in FIGS. 8Aand 8B could be implemented with a photoconductor switch that is lossywhen exposed to light and in this way de-Qs the circuit. See, e.g., Sun,et al., Active Superconducting Devices, filed Jun. 24, 1991, andincorporated herein by reference.

An additional switch technique relies on heat generated by an RFtransmit pulse to switch a superconductor in a circuit out of asuperconducting state. If a superconducting inductor receives enough RFenergy from a transmit pulse, the resultant electrical current willexceed the superconducting critical current at one or more points in thecircuit. The material at these points will be heated above the criticaltemperature and will, therefore, become much more resistive. Thisincrease in resistance will limit the RF power absorbed by the inductor.The switching on-to-off and off-to-on times should occur on a time-scaleof microseconds, fast enough to limit absorption from the transmitpulse, while recovering in time to receive the RF signal. The circuitcan include one or more of these switches by designing points withreduced critical current, e.g. by narrowing the line-width or bydamaging the superconductor within a restricted area.

Other techniques to, for example, raise the temperature of asuperconductor above its T_(c) or to add capacitance or inductance maybe advantageously employed.

Test Data

Several high temperature superconducting coils (“HTS coils”), as shownin FIG. 3 and described above, having resonant frequencies near 2.74 MHzwere manufactured and tested.

One of the HTS coils was designed to be soldered onto a standard Toshibaelectronic control board for MRI coils. The transition from wire-bondsto solder pads was made by coating small LaAlO₃ pieces with gold. Thesepieces were glued on the edge of the coil, and contacts to both ends ofthe inductor were made with HTS crossovers and wire-bonds.

Two HTS inductors were also fabricated and attached to BNC cables. Theseinductors were used to inductively couple to the HTS resonant coils,thus tuning the coils and allowing for signal detection without the needto solder wires to the resonant coil.

A set of dewar assemblies was also designed and fabricated. Thenonmetallic dewars allowed the HTS coils to be held in a liquid nitrogenbath, separated from a room-temperature sample by a few millimeters. Thedewar assemblies were designed either for inductive coupling orsoldering to the Toshiba electronics board. In the inductive couplingmode, an HTS inductor could be held rigidly at any desired distance fromthe resonant HTS coil. A sliding mechanism with a LaAlO₃ piece wasattached to the resonant coil to vary the capacitance of the tuningcapacitor in the liquid nitrogen bath. Additional LaAlO₃ holders couldbe added to press LaAlO₃ pieces against the main capacitor. The resonantcoil could also be coupled to the Toshiba electronics board by attachingthe board to the dewar assembly and soldering wires from the board tothe wire-bond/solder transition pads.

Imaging experiments were carried out. Coil #2L1256 was mounted in theinductive coupling mode and had a measured Q of approximately 1,160. Animage was made of an oil phantom (a loss-less test sample) and comparedto an image made by a similar copper coil (Q≈450) which was alsoinductively coupled. The HTS coil gave a better signal-to-noise ratio(SNR) of roughly sixty percent.

Coil #2L1349 was attached to the Toshiba electronics board. Beforeattaching the board the coil Q value was approximately equal to 1,900.The addition of the board lowered the Q value to about 1,200. Placingthe coil in the magnetic field of the MRI machine (640 Gauss) loweredthe Q by a very small amount. In comparison to a similar copper coil,also attached to an electronics board and cooled in liquid nitrogen, theHTS coil gave an improvement in SNR of about sixty percent for an oilphantom, thirty percent for a saline phantom, and thirty percent for ahuman wrist. This is a significant increase in performance.

Images were also made using a room temperature copper coil (Q≈190)soldered to the electronics board and pressed directly against a salinephantom and a human hand. Because of the insulation thickness, the HTScoil was further from the body, giving it a disadvantage in SNR whencompared to the warm copper coil. For the HTS coil on the Toshibaelectronics board, the SNR at the surface of the body was 20% lower thanthe warm copper coil. However, the effect of the insulation spacing isreduced with increasing distance into the body, and at a depth of 2inches the HTS coil had a 50% advantage in SNR over the warm coppercoil. The insulation thickness will be even less important for largerdiameter coils, and we also anticipate that thinner insulation layerswill be used in future dewar designs.

Coil #2L1335 was mounted in an inductive coupling mode. The measured Qwas about 3,000, but the images had dark bands characteristic of spinover-flipping, and a good measurement of SNR was not available. Theresonant coil and the main RF transmission coil were not completelydecoupled. This problem can be easily remedied with, for example, adecoupling switch or de-Qing switch as previously described.

HTS coils as shown in FIG. 4 and described above were also tested. Thesecoils exhibited Q values of up to 8,260 at 5.8 MHz.

Although the invention has been described with respect to specificpreferred embodiments, many variations and modifications may becomeapparent to those skilled in the art. It is therefore the intention thatthe appended claims be interpreted as broadly as possible in view of theprior art to include all such variations and modifications.

What is claimed is:
 1. A method for tuning a resonant circuit includinghigh temperature superconducting components comprising the steps of:fabricating a resonant circuit, including high temperaturesuperconducting material on a substrate, having both capacitance andinductance; and scribing the resonant circuit so as to affect thecharacteristics of the resonant circuit.
 2. The method according toclaim 1, wherein the high temperature superconducting material is athallium-based superconductor.
 3. The method according to claim 1,wherein the high temperature superconducting material is a yttrium-basedmaterial.
 4. The method according to claim 1, wherein the hightemperature superconducting material is a bismuth-based material.
 5. Themethod according to claim 1, wherein the step of scribing producesdynamic tuning of the resonant circuit.
 6. The method according to claim1, wherein the high temperature superconducting material comprises anepitaxial thin film superconductor.
 7. The method according to claim 1,wherein the substrate comprises a dielectric wafer.
 8. The methodaccording to claim 7, wherein the dielectric wafer is equal to or lessthan 2 inches in diameter.
 9. The method according to claim 1, whereinthe resonant circuit comprises a high temperature superconductingcapacitor coupled to a high temperature superconducting inductor. 10.The method according to claim 9, wherein the high temperaturesuperconducting capacitor is fabricated monolithically on the samesubstrate as is the high temperature superconducting inductor.
 11. Themethod according to claim 1, wherein the resonant circuit is used topick-up radio frequency radiation.
 12. A method of tuning a hightemperature superconducting resonator comprising the steps of: providinga substrate having a planar surface; providing a high temperaturesuperconducting inductor on the planar surface of the substrate;providing a high temperature superconducting capacitor on the planarsurface of the substrate, the capacitor being electrically connected tothe high temperature superconducting inductor; providing a tuning bodyadjacent to the planar surface; and altering the relative position ofthe tuning body and the planar surface so as to tune the resonator. 13.The method according to claim 12, wherein the tuning body is adielectric.
 14. The method according to claim 12, wherein the tuningbody is a conductor.
 15. The method according to claim 12, wherein thestep of altering the relative position of the tuning body and the planarsurface produces dynamic tuning of the resonator.
 16. The methodaccording to claim 12, wherein the high temperature superconductingcapacitor has an interdigitated structure.
 17. The method according toclaim 12, wherein the high temperature superconducting inductor and thehigh temperature superconducting capacitor comprise an epitaxial thinfilm superconductor.
 18. The method according to claim 12, wherein thesubstrate comprises a dielectric wafer.
 19. The method according toclaim 18, wherein the dielectric wafer is equal to or less than 2 inchesin diameter.
 20. The method according to claim 12, wherein the hightemperature superconducting capacitor is fabricated monolithically onthe same substrate as is the high temperature superconducting inductor.21. A method of tuning a high temperature superconducting resonatorcomprising the steps of: providing a substrate having a planar surface;providing a high temperature superconducting resonator patterned from ahigh temperature superconducting thin film; providing a tuning bodyadjacent to the planar surface; and dynamically altering the relativeposition of the tuning body and the planar surface so as to tune thehigh temperature superconducting resonator.
 22. The method according toclaim 21, wherein the tuning body is a dielectric.
 23. The methodaccording to claim 21, wherein the tuning body is a conductor.
 24. Themethod according to claim 21, wherein the high temperaturesuperconducting resonator comprises an epitaxial thin filmsuperconductor.
 25. The method according to claim 21, wherein thesubstrate comprises a dielectric wafer.
 26. The method according toclaim 25, wherein the dielectric wafer is equal to or less than 2 inchesin diameter.
 27. The method according to claim 21, wherein the hightemperature superconducting thin film is thallium-based.
 28. The methodaccording to claim 21, wherein the high temperature superconducting thinfilm is yttrium-based.
 29. The method according to claim 21, wherein thehigh temperature superconducting thin film is bismuth-based.
 30. Amethod of tuning magnetically coupled high temperature superconductingresonators comprising the steps of: providing a first high temperaturesuperconducting resonator; providing a second high temperaturesuperconducting resonator in proximity to the first high temperaturesuperconducting resonator so as to magnetically link the first hightemperature superconducting resonator to the second high temperaturesuperconducting resonator; and tuning one of the first and second hightemperature superconducting resonators.